Isotope separation by laser means

ABSTRACT

A process for separating isotopes by selective excitation of isotopic species of a volatile compound by tuned laser light. A highly cooled gas of the volatile compound is produced in which the isotopic shift is sharpened and defined. Before substantial condensation occurs, the cooled gas is irradiated with laser light precisely tuned to a desired wavelength to selectively excite a particular isotopic species in the cooled gas. The laser light may impart sufficient energy to the excited species to cause it to undergo photochemical reaction or even to photoionize. Alternatively, a two-photon irradiation may be applied to the cooled gas to induce photochemical reaction or photoionization. The process is particularly applicable to the separation of isotopes of uranium and plutonium.

The invention described herein was made in the course of, or under, acontract with the U.S. ATOMIC ENERGY COMMISSION.

REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of application Ser. No.270,800 now abandoned, also entitled "Isotope Separation by LaserMeans," and filed July 7, 1972.

BACKGROUND OF THE INVENTION

The invention relates to a method of isotope separation based onselective excitation of isotope species and more particularly to amethod wherein the selective excitation is initiated by laser means.

For various nuclear applications it is exceedingly important that thefissile isotopes ²³⁵ U and ²³⁹ Pu be separated from or strongly enrichedin admixtures with other isotopes of uranium and plutonium,respectively. Presently, the only economically viable method forproducing uranium enriched in ²³⁵ U is the gaseous diffusion processwhich requires very large capital investment and tremendous plantfacilities. There is presently no practical scheme available forseparating ²³⁹ Pu from ²⁴⁰ Pu, a separation which is highly desirablefor certain military applications.

With the advent of lasers tunable to very narrow frequencies over a widerange of the spectrum, it has become apparent that by controlling thespectral response of the interaction of light with matter, it ispossible, in principle, to produce selective reactions that can changethe composition and properties of the matter. The conditions required toachieve such selectivity are: (1) high monochromaticity of the excitinglight; (2) the selectivity of the primary process of light interactionwith the matter (the existence of narrow nonoverlapping absorptionlines); and (3) conservation of the induced selectivity in successivephysical and chemical processes. See, e.g., R. V. Ambartzumian and V. S.Letokhov, "Selective Two-Step (STS) Photoionization of Atoms andPhotodissociation of Molecules by Laser Radiation,38 11 Applied Optics354 (1972).

Laser art has advanced sufficiently that tunable lasers havingbandwidths narrower than 0.0002 cm⁻¹ are available so that the firstcondition is completely satisfied. Efficient amplification of narrowlytuned infrared oscillators can be accomplished with high pressureelectron beam controlled electric discharge gas lasers. Such a schemeallows narrow bandwidth with high overall electrical efficiency. Highoverall efficiency can be obtained by use of parametric oscillator andphase matching techniques for tuning efficient visible and ultravioletlasers such as lead atom, copper atom, and Xe₂ lasers. Tunable dyelasers have sufficiently narrow bandwidths in the range 3600 to 7200 A,although their electrical efficiency is lower.

The second and third conditions present substantial problems. Forexample, in principle the second condition can be met by the interactionof precisely tuned laser light provided that there exist certaindiscrete electronic and vibrational transitions of matter in the gaseousphase. Even if discrete transitions exist, it is frequently difficult toascertain in a gaseous species which transitions are appropriate forselective interaction with tuned laser light.

Once selective excitation has been made to occur, there are numerousprocesses by which the selectivity may be lost. A primary loss mechanismis collisional energy transfer between molecules. Thus if the thirdcondition is to be achieved, it is highly desirable that the selectivelyexcited species be transformed to a stable or metastable state. Onemeans by which the selectivity can be stabilized is throughphotoionization or photodissociation of an excited species. A problem,however, is that photoionization or photodissociation may not themselvesbe selective.

It is known in the art that the stabilizing effect of eitherphotoionization or photodissociation may be used advantageously if theyare separated from the selective excitation step through use of photonsor light quanta of differing energies hν₁ and hν₂. Photons of energy hν₁excite a certain state of the discrete energy spectrum in a particularspecies, and photons of energy hν₂ photoionize or photodissociate theexcited species. The energies of the photons satisfy the followingconditions:

    hν.sub.1 +hν.sub.2 >E.sub.i,E.sub.d

    hν.sub.2 <E.sub.i,E.sub.d

where E_(i) is the photoionization energy of an atom or molecules fromthe ground state and E_(d) is the photodissociation energy of a moleculefrom the ground state.

The art indicates that this two-step process, or two-photon process asit is also known, is applicable to the separation of isotopes. Aprerequisite for such separation is the existence of a suitable isotopeshift in the absorption spectra of the element or one of its compoundsso that only one isotopic species is excited by the tuned light.

In U.S. Pat. No. 3,443,087, issued May 6, 1969, Robieux et al. reveal aprocess for ionizing selectively a gaseous compound of an isotope whichis a part of a mixture of isotopes which comprises irradiating themixture of isotopes with light of two different wavelengths in twosteps, the first irradiation by light of one wavelength serving toselectively excite the molecules of one isotope and the second by lightof another wavelength serving to ionize the excited molecules. Theionized molecules are then subjected to electric or magnetic fields or acombination thereof to deflect them away from the un-ionized isotopiccompound.

Using a first irradiation of infrared light and a second irradiationwith ultraviolet light, Robieux et al. indicate that ²³⁵ UF₆ and ²³⁸ UF₆may be separated according to the process of their invention. Therationale behind their two-photon process is that finely tuned energyavailable from absorption in the infrared region of the spectrum willselectively excite one of the uranium isotopes, preferably the ²³⁵ U,but is inadequate to excite the isotopic compound which is absorbing itsufficiently to produce ionization. Line breadths in the ultravioletspectral region, where there is sufficient energy to produce ionization,are larger than at lower frequencies so that it is much more difficultto achieve the requisite selective absorption in this region of thespectrum. That is, although photoionization can readily be produced byultraviolet light, it is not likely to be selective. Through use of thetwo-step absorption process, one isotopic species is selectively excitedby the infrared and then a sufficient amount of energy is provided bythe ultraviolet (which is absorbed by both species) to just drive theexcited isotopic compound past the ionization threshold, whereas theisotopic compound that remained in the ground state during the infraredirradiation is not sufficiently excited by the ultraviolet to be ionizedeven though it absorbs to substantially the same degree.

Reasonably sharp isotope shifts have been identified for uranium and itscompounds, but at either very high or very low temperatures. The veryhigh temperatures have been necessary for elemental uranium.Unfortunately, even at 1600° C. uranium has a vapor pressure of only 1micron, which is much too low to obtain any reasonable light interactionwith the vapor. Thus a substantially higher temperature is required, andan isotope separation process based on the use of elemental uranium asthe feed material does not therefore appear practical. Cesium uranylchloride (CsUO₂ Cl₄) and cesium uranyl nitrate (CsUO₂ (NO₃)₃) enrichedin ²³⁵ U have shown an isotopic shift of 1.62 cm⁻¹ at 20° K. While thespectral lines are sharp at 20° K., they become broad at 77° K. andcannot be resolved at higher temperatures. At the low temperatures atwhich the lines are defined, however, these compounds exhibitessentially no vapor pressure.

Certain isotopic shifts in the infrared spectrum of UF₆ at roomtemperature have been determined by measurements on separated samples of²³⁸ UF₆ and ²³⁵ UF₆. The 623 cm⁻¹ ν₃ (F_(1u)) band shows a measuredshift of 0.55 cm⁻¹. Measurements on the other infrared bands indicate ashift of 0.1 to 0.2 cm⁻¹ for the ν₄ (F_(1u)) vibration, the only otherof the six vibrations which should show a nonzero isotope shift. Thesemeasured shifts are gross in nature, however, and no fine line spectrawere resolved.

Although Robieux et al. in U.S. Pat. No. 3,443,087 state that a chemiclreaction may be used to separate the isotopes, they give no example ofwhat chemical reactions will suffice or how such chemical reactionsmight be brought about. They consequently make no claims with respect tochemical separation. In a recent report, R. C. Farrar, Jr. and D. F.Smith review the literature dealing with photochemical means for isotopeseparation, with particular emphasis on the separation of uraniumisotopes. See "Photochemical Isotope Separation as Applied to Uranium,"Union Carbide Oak Ridge Gaseous Diffusion Plant Report K-L-3054, Rev. 1(Mar. 15, 1972). Although photochemical dissociation of UF₆ would haveadvantages over photochemical reactions involving two molecular species,Farrer et al. do not devote any discussion to it.

SUMMARY OF THE INVENTION

Selective excitation of isotopic series of a gaseous compound by laserlight forms the basis of an efficient isotope separation method. Themethod is straightforward for separating isotopes of elements which formcompounds that are volatile at low temperature, that is, a temperaturesuch that RT is less than E*, the energy of the lowest vibrational stateof the molecule. However, to separate isotopes of elements that do notform compounds volatile at low temperatures, it is necessary to preparethe compound in a supersaturated gaseous state in order to obtain thefollowing advantages of a low temperature environment: (1) slow energytransfer and therefore no or greatly reduced scrambling, (2) simpleinfrared spectrum because of depopulation of vibrational states androtational states, (3) low adverse chemical reactivity, and (4) goodspectroscopic separation factors because of sharpened infrared, visible,or ultraviolet spectrum.

We have found that supersaturated gas of the desired low temperature canbe readily prepared by adiabatic expansion of a volatile compoundthrough a converging-diverging nozzle of the type well known in the gasdynamic laser art. The gas is then irradiated with a precisely tunedlaser to provide selective excitation of an isotopic species beforesubstantial condensation of the gas occurs. The excitation may besufficient to produce photochemical reaction or photoionization of theexcited species. The photochemical reaction may take the form ofphotodissociation of the excited molecules or reaction of the excitedmolecules with a second molecular species. Alternatively, the two-photonprocess may be applied to the supersaturated gas to provide therequisite selective photoionization or photochemical reaction. In thecase of photoionization, electrical or magnetic means or a combinationthereof are provided to separate the ionized from the un-ionizedspecies. In an embodiment employing photochemical reaction, physical orchemical means are provided for separating the isotopic product of thereaction from the unreacted isotopic species.

The method of this invention is applicable to any isotope separationscheme using selective laser excitation of isotopic species wherein amore sharply defined isotopic shift results from precooling. It hasparticular utility in the separation of isotopes of uranium andplutonium using compounds such as UF₆, UCl₄, UBr₄, and PuF₆.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the vibrational energy distribution in UF₆ at 300° K.

FIG. 2 shows a nozzle suitable for adiabatically expanding a gaseouscompound in the process of this invention.

FIG. 3 shows (a) the theoretical shape for a P (or R) branch absorptionband of a spherical top molecule such as UF₆, and (b) certain curves ofthe spectroscopic separation factor for ²³⁵ UF₆ and ²³⁸ UF₆ at severaltemperatures of interest.

FIG. 4 is a calculated infrared absorption spectrum for the ν₃ state innatural UF₆, i.e., UF₆ containing 0.7% ²³⁵ U, at 50° K. with a ²³⁸ UF₆-²³⁵ UF₆ isotope shift of 0.68 cm⁻¹.

FIG. 5 shows the vibrational energy distribution in UF₆ at 50° K.

FIG. 6 shows measured ultraviolet absorption of UF₆.

FIG. 7 shows an absorption mechanism for ultraviolet in UF₆ which leadsto photodissociation.

FIG. 8 is a comparison of calculated and measured ultraviolet absorptiondata for UF₆.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In molecules containing different isotopes the classical vibrationfrequencies differ according to

    |Δν|=1/2(Δμ/μ)ν

where ν is the vibration frequency, and μ is the reduced mass of themolecule. The reduced isotopic mass difference Δμ is nonzero only if theisotopic atoms are in motion during the vibration. The resultantdifference in frequency, Δν, is called the isotope shift. Laser isotopicseparation is possible because the frequency purity of laser light issufficient to be reasonantly absorbed by one isotopic species withoutaffecting other nonreasonant isotopes. In any isotope separation processbased on the selective excitation of isotopic species by tuned laserlight, it is essential that the isotopic shift be as sharply delineatedas possible.

As used in this application, the term "laser light" includes coherentradiation in the ultraviolet, visible, and infrared portions of thespectrum. By selective excitation is meant the tuning of the laser lightfrequency to effect absorption by an optical absorption state of oneisotope only.

To obtain very sharp spectral features, it is highly preferable that theisotopic mixture which is to undergo separation be in the gaseous statewhen it is irradiated with the tuned laser light. Gaseous mixturespermit efficient interaction of the laser light with the desiredisotopic species while at the same time limiting the possibility thatselectively excited molecules will undergo scrambling, that is, transferof their excitation to nonexcited species. It will be readily apparentthat scrambling reduces the efficiency of isotope separation and ifsufficiently severe may prevent any separation at all.

The separation of isotopes of heavy elements is, in general, moredifficult than separation of isotopes of light elements. The isotopeshifts of optical absorption lines are much smaller for compounds ofheavy elements, and since most of the gaseous compounds of heavyelements are polyatomic, there exist a very large number ofvibration-rotation states closely spaced in frequency. Selective lightabsorption is further complicated for heavy elements because attemperatures at which their compounds are gaseous, a large number ofvibration-rotation states are already excited.

For example, the room temperature infrared spectrum of UF₆ vaporcorresponds to a combination rotational-vibrational transition. Itincludes many lines that have never been resolved. A primary reason forthe gross nature of the isotope shifts in UF₆ at room temperature is theexistence of so-called "hot" bands in the spectrum. Because the threebending vibrations occur at low frequencies, the excited bendingvibration states are sufficiently populated at room temperature thatalmost none of the molecules are in the ground state. Also, due toanharmonic effects, the "hot" bands do not precisely coincide with theground state bands. As a result, there are many more lines in thespectrum than would otherwise be present if the "hot" bands either didnot exist or were not appreciably populated.

FIG. 1 shows the extent of UF₆ "hot" bands populated at 300° K. The plotis the contribution dQ_(i) for a particular vibrational level i to thetotal vibration partition function for equilibrium at 300° K., where##EQU1## where D(i) is the degeneracy of the i^(th) state, ν_(i) are thefrequencies of the six fundamental vibrational modes, and v(i) are theoccupation quantum numbers. It should be noted that only 0.4% of themolecules are in the ground vibrational state and that more than half ofthe molecules are excited to energies greater than 1200 cm⁻¹. There are13,000 levels which are excited. Degeneracies as high as 22,000 areachieved. It is readily apparent that tuning a laser to operate betweentwo individual states at room temperature is an exceedingly difficulttask and there is no indication that it can in fact be done.

Spectral separation and resolution of isotopically shifted absorptionlines or narrow bands may, however, be much improved by having theabsorbing material at the lowest feasible temperature. In fact,sufficient cooling results in clear-cut spectrum simplification. Byspectrum simplification is meant essentially the removal of overlappingabsorption features in order to isolate selectable isotopic absorptionstates. But cooling alone is not enough to achieve an effective isotopeseparation process based on selective excitation of isotopic species. Atthe same time, while the material density must not be so high as todegrade the resolution by pressure broadening, it must be high enough toyield acceptable absorption in reasonable optical path length. When theequilibrium vapor pressure of the optically absorbing compound isunacceptably small at the desired low temperature, an optimum state ofthe absorber, i.e., some reasonable density at the desired temperature,can be achieved by adiabatically expanding the gas. This can be done bya supersonic expansion which results in a state of very highsupersaturation at low temperature in the high velocity gas stream.

When a fluid is expanded to supersonic velocities, a large reduction inpressure is required. Since the flow derives its energy from theinternal energy and random motion of the fluid, a large reduction in thebulk fluid temperature also ensues. Apparatus for producing such a fluidexpansion ranges from ordinary nozzles to molecular beam nozzles. Apreferred means of producing the requisite adiabatic expansion is aconvergent-divergent slit nozzle of the type well known in thegas-dynamic laser art. A nozzle of this type is preferred over amolecular beam apparatus because the number density of molecules to beisotopically separated in the cooled region is much greater. A molecularbeam apparatus consists of a nozzle with expansion walls normal to theflow direction and must be operated at very high pressure ratios. Insome circumstances, it may be useful to utilize the collisionless regimeof a molecular beam rather than the collision dominated conditions ofwalled nozzle expansion systems. For example, there may be situations inwhich the time between molecular collisions in the expanded fluid mustbe long to avoid scrambling before separative processes occur betweenthe laser-excited and unexcited molecules.

The threshold condition for the onset of condensation in the flow willoccur for some critical pressure at the inlet to the nozzle, below whichsuccessful operation can be achieved over a range of pressures.Condensation of the cooled gas in the light interaction and absorptionzone is undesirable in that substantial light scatter may occur andscrambling is produced, i.e., energy exchange occurs between excited andunexcited isotopic species. The residence time of the flowing gas can bemade of short duration by reducing the nozzle length insofar as this iscompatible with uniform expansion efficiency. Even though condensationin the light interaction and absorption zone is undesirable, it will beapparent that some degree of condensation may be tolerated despite thereduction in efficiency of the separation process which it may produce.The amount of condensation which will still permit effective isotopeseparation depends in large measure on the particular molecular andisotopic species involved.

The cooling that can be achieved by the adiabatic expansion of a singlegas is readily determined by the well-known gas-dynamic relationship

    T/T.sub.o =(P/P.sub.o).sup.(γ-1)/γ

where the subscript zero denotes pre-expansion conditions and γ is theratio of specific heats. Temperatures can be achieved by this techniquewhich are considerably below the condensation point, and the gas willremain supersaturted at high Mach numbers. The lower limit attainable bythis method depends upon the nuclear condensation of the particular gas.If the gas containing the isotopic species to be separated has a low γ(e.g., UF₆ has a γ of 1.065), it can be cooled substantially more by theaddition of a so-called carrier gas. The carrier gas should have a highγ in order to maximize the effective γ of the mixture and therebyminimize T/T_(o) for a given P/P_(o) ratio. The monatomic gases (noblegases) have γ=1.67 and helium in particular also has a condensationpoint of only 4°K., making it the optimal carrier gas. Thus for maximumcooling the nozzle should be fed with helium as the carrier gas and theabsorbing compound admixed in lesser concentration.

The adiabatic expansion is readily accomplished by means of contouredsupersonic nozzles of the type well known in the gas dynamic laser art.Such a nozzle, having a slit configuration, is shown in FIG. 2. Agaseous compound containing a mixture of isotopes is introduced intoplenum chamber 1 and allowed to flow supersonically through throat 2.Diverging region 3 of the nozzle is so designed to allow optimumexpansion of the gas to some uniform pressure which obtains in constantarea duct 4. In duct 4 the gas is in a supersaturated state and highlycooled. Preferably, little or no condensation occurs in duct 4 beforethe gas enters exhaust 5. The cooled gas in duct 4 is easily irradiatedtransversely by means of laser beam 6 from laser 7. The optical pathlength of irradiating beam 6 may be governed by the length of throat 2and the number of mirrors 8 by which light beam 6 is reflected back andforth in duct 4. Factors which determine a reasonable optical pathlength are the photon absorption cross section and (adjustable) gasnumber density.

Laser 7 is finely tuned so that beam 6 selectively excites moleculescontaining only a particular isotopic species. In various embodiments ofthe process of this invention, beam 6 may be sufficiently energetic to(1) induce photochemical reaction of the excited molcules with a secondgaseous compound mixed with the compound containing a mixture ofisotopes, (2) photodissociate the excited molecules, or (3) photoionizethe excited molecules. In cases (1) and (2), physical and/or chemicalmeans may readily be used to separate the reaction product containingthe particular isotopic species from the unreacted gas. In case (3),deflecting means such as an electric field, a magnetic field, or acombination thereof may be used in duct 4 to deflect ionized moleculescontaining the particular isotopic species away from the un-ionizedmolecules exhausting from duct 4. In still other embodiments, thetwo-photon process may be used to produce essentially the samephotochemical reaction or photoionization. In the two-photon process, anadditional light source is used to irradiate the cooled gas in duct 4 ata different frequency than that of laser 7. It will be apparent thatthis second irradiation can easily be accomplished with a nozzle of thetype shown in FIG. 2.

The advantageous aspects of adiabatic cooling as applied to isotopeseparation based on selective excitation of isotopic species may bedemonstrated with regard to mixtures of ²³⁵ UF₆ and ²³⁸ UF₆. Uraniumhexafluoride is the only uranium compound having any substantial vaporpressure at or near room temperature. Typically, this compound exhibitsa vapor pressure of about 100 Torr at room temperature, but at 75° K. orless, where thermal bands are depopulated, it has a vapor pressure thatis immeasurably small (est. 10⁻²⁶ mm Hg).

The enhanced degree of resolution of the absorption bands in a mixtureof the two isotopic molecules obtained by cooling the gas isdemonstrated in FIG. 3. As the rotational temperature is decreased, theabsorption envelopes get sharper and more intense, such that the peak ofthe R-branch of ²³⁵ UF₆ is displaced from the peak for ²³⁸ UF₆ and is infact coincident with the tail of the ²³⁸ UF₆ absorption envelope. Underthis condition the ²³⁵ UF₆ will be preferentially excited by light whosewavelength coincides with the position of the ²³⁵ UF₆ peak, since the²³⁸ UF₆ will absorb to a much lesser extent.

In FIG. 3, the dimensionless parameter y gives the relative intensity asa function of x for the R-branch, and x represents a given wave numberfor excitation. Parameter y is given by

    y=α.sub.x /α.sub.1

where α_(x) is the absorption coefficient at wave number x and α₁ isgiven by the relationship

    α.sub.1 =(-1/lP)log(I/I.sub.o)≈5×10.sup.-2 (300/T).sup.3/2 Torr.sup.-1 cm.sup.31 1.

The parameter y/y' is the selectivity for a given position in therotational envelope (i.e., at a given x) and δ is the ratio of theisotope shift to the envelope width and is a function of the rotationaltemperature according to the expression ##EQU2## For example, if amixture of ²³⁵ UF₆ and ²³⁸ UF₆ is irradiated with a laser tuned to theportion of the ²³⁵ UF₆ R-branch corresponding to x=2 and the temperatureis 50° K., the proportion of ²³⁵ UF₆ molecules in the excited populationis ˜5 times as great as the proportion in the unexcited population. Ifthe temperature is 20° K., this number is increased to ˜20. Startingwith natural UF₆ at 50° K., about 3.5% of the excited molecules willcontain ²³⁵ U. FIG. 3 shows in quantitative terms the advantage of lowtemperatures and the trade-off between good separation factors and poorabsorption as x is increased.

The strongest absorption band for UF₆ is at 626 cm⁻¹ (16.0μ). Theliterature indicates that this band has a 0.55 cm⁻¹ isotope shift atroom temperature, i.e., 300° K., as measured at the peak of the broadabsorption contour. There is nothing in the prior art that reveals anymeasurement of the isotope shift in gaseous UF₆ at lower temperatures.The present inventors have found that at 228° K. the absorption contouris substantially sharpened and the isotope shift is measured as 0.68cm⁻¹. There are three other bands that have about the same isotope shiftbut are ˜100 times weaker. They are at 1294 cm⁻¹ (7.73μ), 1151 cm³¹ 1(8.69μ) and 825 cm⁻¹ (12.12μ). Any other bands with suitable isotopeshifts will be at least 20 times weaker yet. At other than the 16μ band,the optical path length must be about a meter or more. The peakabsorption coefficient in the above units is proportional to T^(-3/2),so lower temperatures give somewhat larger peak intensities.

FIGS. 4 and 5 further demonstrate certain of the advantageous effects ofcooling in enhancing the degree of resolution of the absorption bands ina mixture of two isotopic species. FIG. 4 presents the calculatedinfrared absorption versus wave-number for the ν₃ fundamentalvibrational state in the UF₆ gas phase at 50° K. with a ²³⁸ UF₆ -²³⁵ UF₆isotope shift of 0.68 cm⁻¹. The P, Q, and R transitions are indicated.The computer code which produces this calculation contains thespectroscopic terms obtained from UF₆ spectra and gives excellent fitsto infrared absorption data for ν₃ at 373° K., 300° K., and 238° K. FromFIG. 4, it is obvious that selective absoprtion of either ²³⁸ UF₆ or ²³⁵UF₆ at 50° K. can be accomplished by tuning a laser to the properfrequency. In this regard, it should be noted that gas lasers areavailable which have a nominal width of ˜0.005 cm⁻¹. In order to workwith UF₆ in as many initial rotational states as possible, it isadvantageous to operate at the Q-branch absorption. In the enlargedporton of FIG. 4, the solid line represents the Q-branch while thedotted line is the R-branch. FIG. 5 shows the dramatic effect on the"hot" bands of UF₆ when it is cooled to 50° K. More than 93% of themolecules are in the ground vibrational state. The remainder are in thefirst excited state, the ν₆ fundamental.

With a nozzle such as that shown in FIG. 2 providing an expansion ratioof 20:1, and natural UF₆ at room temperature (300° K.) in the plenumwith a UF₆ pressure of 100 Torr and a helium pressure of 900 Torr,expansion through the throat and diverging region can in principleprovide a supersaturated gas in the constant area duct at a pressure(combined He and UF₆) of 2 Torr and a temperature of about 30° K. Thedensity of the gas in the constant area duct is about 5×10¹⁶molecules/cm³, which allows a quite reasonable optical path length.

The cooling effect of a nozzle such as that shown in FIG. 2 has beendemonstrated. A gas mixture consisting of 5% natural UF₆ and 95% He at atemperature of 300° K. and a pressure of about 1450 Torr was expandedthrough a slit nozzle having an area ratio of 22 to product asupersaturated gas in the constant area duct at a pressure (combined Heand UF₆) of 4 Torr and a temperature of about 48° K. There was noevidence of condensation in the constant area duct.

It is in principle possible to use laser excitation to produceseparation of uranium isotopes from supersaturated gaseous UF₆ at 50° Kby means of photodissociation, photoionization, photoreaction, andphotodeflection techniques. Photodeflection is no part of the presentinvention; however, the other techniques are within the ambit of theinvention. A preferred technique for separating uranium isotopes usingsupersaturated gaseous UF₆ is a two-photon photodissociation process. Inone embodiment of this process, a mixture of He and UF₆ is expandedthrough a supersonic contoured slit nozzle of a type well known in thegas dynamic art to high Mach numbers with a very low local temperature,i.e., on the order of 50° K., while yet maintaining a UF₆ gas pressureat which reasonable optical path lengths are possible. Thesupersaturated gaseous UF₆ thus produced is then irradiated with a firstlaser light in the infrared portion of the spectrum and a second laserlight in the ultraviolet portion of the spectrum. This two-photonirradiation may be used to produce selective disassociation of a UF₆molecule according to the following two steps.

    UF.sub.6 +hν.sub.1 →.sup.235 UF.sub.6 * (selective excitation) (1)

    .sup.235 UF.sub.6 *+hν.sub.2 →.sup.235 UF.sub.5 +F (photodissociation)                                       (2)

It will be readily apparent that by proper tuning of the infrared laser(hν₁), molecules containing ²³⁸ U can be selectively excited rather thanthose containing ²³⁵ U. Uranium pentafluoride is a stable solid that canreadily be removed from the light interaction and absorption region ofthe flowing UF₆ by physical means, e.g., by filtering, or throughsettling traps, or electrostatic precipitation.

The dissociation energy for the gas phase dissociation of UF₆ to UF₅ hasnot been heretofore reported in the literature but has been calculatedby the present inventors as

    UF.sub.6 →UF.sub.5 +F, ΔH.sub.f (g)=+76 kcal/mole.

This calculated dissociation energy indicates that a wavelength of 3750A or shorter will cause dissociation. To substantiate this, a nitrogenlaser at 3371 A was used to irradiate a cell of UF₆ at 50 Torr and 301°K. The pressure was monitored to a precision of 10⁻³ Torr. Under thelaser irradiation the pressure decreased at a rate consistent with aphotolysis (photodissociation) cross section of about 10⁻²¹ cm². A finewhite powder precipitated from the gas. It was analyzed by x-raycrystallography and shown to be UF₅. The quantum yield for this processat the 3371 A wavelength was found to lie between 0.01 and 1.0. Thus, bycooling natural UF₆, preferably to 75° K. or less, and using a finelytuned laser, it is possible to selectively excite one isotopic speciesand with a sufficient amount of excitation energy to cause theselectively excited molecules to dissociate.

Indeed, there is substantial reason to believe that the second photon,i.e., the ultraviolet hν₂, of the two-photon dissociation will not evenbe absorbed by those molecules that have not been selective excited bythe first photons, i.e., the infrared hν₁. The ultraviolet absorption ofUF₆ at room temperature is known in the art to be as shown in FIG. 6.There is measurable absorption only for wavelengths shorter than 0.45 μmin the ultraviolet. The photolysis wavelength indicated on FIG. 6 is thewavelength at which or below which the calculated dissociation energyindicates that dissociation will occur. It is apparent that thedissociation threshold for UF₆ is in the vicinity of the absorption dataof FIG. 6. But the optical absorption is immeasurable throughout thenear infrared and visible ranges, which indicates that the transitionprobabiity to the high lying vibrational states is very low. The onsetof absorption at 4000 A suggests that a different mechanism, i.e., theopening of the dissociation channel, is responsible for the absorption.A calculation which tests this suggestion is outlined in FIG. 7. If asharp absorption (dissociation) threshold exists, an optical wavelengthscan will first promote the highest energy "hot band" states todissociation, then successively operate on the lower lying states as thelight is tuned to shorter wavelengths. Thus, the shape of the opticalabsorption may be predicted by simply integrating the partition functionat 300° K. (see FIG. 1) from the right, normalizing to the point wherethe total integral of states dominate. FIG. 8 compares the results of acalculation of this type with previously measured absorption dataobtained by Young (cf. Farrar et al., op. cit.p. 35). The fit to thedata is convincing proof that a dissociation mechanism accounts for theabsorption. It is thus apparent that since absorption in this spectralrange is dependent on matching or exceeding the dissociation threshold,the second photon of a two-photon dissociation should not even beabsorbed by those UF₆ molecules not excited by the first photon. Thus,the process should be a very selective one, with the first and secondphotons affecting one isotope only.

Plutonium hexafluoride is a volatile compound quite similar to UF₆ inits spectral properties. It has a boiling point at atmospheric pressureof 62.3° C. and a vapor pressure of approximately 100 mm of Hg at 23° C.The process of this invention is applicable to the separation of ²³⁹ Pufrom ²⁴⁰ Pu in a mixture of ²³⁹ PuF₆ and ²⁴⁰ PuF₆ in substantially thesame fashion as has been described for the separation of uraniumisotopes utilizing gaseous UF₆.

Although the examples given herein have been limited to the separationof isotopes from UF₆ and PuF₆ isotopic mixtures, respectively, with theisotopic mixtures initially being at or near room temperature, it willbe apparent that the process of this invention is not limited either tothe conditions or to the compounds described. For example, in theseparation of uranium isotopes, it may well be desirable to begin withisotopic compounds containing two to five atoms which are volatile onlyat temperatures substantially above room temperature and expanding themto a supersaturated state which is much cooler than the melting point,but which may still be well above room temperature. Examples of suchcompounds and their melting points are

    ______________________________________                                        Compound          Melting Point, °C.                                   ______________________________________                                        US.sub.2          1100                                                        UCl.sub.4         590                                                         UCl.sub.3         842                                                         UF.sub.4          960                                                         UBr.sub.4         516                                                         UI.sub.4          506                                                         ______________________________________                                    

Photochemical isotope separation according to the process of thisinvention is not limited to photolysis, i.e., photodissociation, butalso encompasses the use of other photochemical reactions. Thus, forexample, homogeneous reactions which may be induced by laser excitationof UF₆ include:

    UF.sub.6 *+N.sub.2 F.sub.4 →NF.sub.3 +NF.sub.2 +UF.sub.5

    UF.sub.6 *+SF.sub.4 →SF.sub.5 +UF.sub.5.

Vibrational excitation, denoted by the asterisk, enables the reducingagent to abstract one of the F atoms from the UF₆. By providing anexcess of the reducing agent, it is possible to ensure that the excitedUF₆ chemically reacts before it has a chance to collisionally transferthe vibrational excitation to a different isotopic species.

It will be apparent from the foregoing that a critical feature of thepresent invention is cooling a volatile compound containing a mixture ofisotopic species to simplify the optical absorption spectra and sharpenthe isotope shifts, while at the same time retaining a sufficientmaterial density to allow a reasonable optical path length forinteraction of tuned laser light with the gaseous compound to produceselective photochemical reaction or photoionization of moleculescontaining a particular isotopic species. The cooling substantiallyenhances the ability to produce selective absorption in only oneisotopic species.

What we claim is:
 1. A method of achieving spectrum simplification ofUF₆ while retaining sufficient vapor pressure to allow gas-phase,laser-induced photochemical reaction of said UF₆ to occur with areasonable optical path length which comprises:(a) obtaining said UF₆ ina gaseous state with a carrier gas admixed therewith to form a gaseousmixture; and, (b) adiabatically expanding said gaseous mixture into aregion of uniform pressure by supersonically flowing said gaseousmixture through a convergent-divergent slit nozzle into a constant areaduct.